LTE Advanced is being considered for deployment in the unlicensed spectrum in the 5 GHz band. To do so, licensed spectrum in the range 400 MHz to 3.8 GHz and spectrum in the unlicensed spectrum bands would be aggregated together similar to the way that the licensed spectrum bands are treated in standard LTE Advanced carrier aggregation (CA). Regulatory authorities around the world have defined or are defining a wide variety of requirements so that various systems can inter-operate with incumbent users in the unlicensed bands, notably including WiFi deployments.
For LTE Advanced (LTE Rel10-12) carrier aggregation (CA) or Dual Connectivity, an eNB (E-Utran Node B) may be used. An eNB is connected to a mobile phone network that communicates with mobile handsets (UEs) and that configures a secondary serving cell (Scell) to the UE to provide additional frequency resources (i.e., a secondary carrier or a secondary component carrier (CC)) for communication in addition to the primary serving cell (Pcell). The Scell is typically activated for a UE at the onset of a data burst transmission for the UE and deactivated after the transmission is complete. Activation is done by using an activation command MAC layer control element (MAC CE). Deactivation is done either upon expiry of a deactivation timer or by using an explicit deactivation command MAC CE. Additionally, a UE operating in LTE Advanced (LTE Rel12) network can expect at least discovery signals from the eNB on the Scell carrier while the carrier is deactivated. After receiving an activation command for the Scell, the UE expects cell specific reference signal (CRS) transmission from the eNB on the Scell carrier from the subframe where activation command is received till the subframe where the Scell is deactivated.
For further Advanced LTE (LTE Rel13) Scell, operation on unlicensed carriers continues to be studied. The initial focus of the studies appears to be Scell operation via a CA mechanism. However, it has been perceived that some of the procedures identified for CA can also be reused for dual connectivity (i.e., when the Scell and Pcell belong to different cell groups).
Due to regulatory requirements, and due the need to for Advanced LTE to co-exist with other wireless systems (e.g., Wi-Fi), LTE devices (i.e., UEs and eNBs) need to take the following issues into account while operating on an unlicensed carrier frequency.
First, before transmitting on an unlicensed carrier, the LTE devices (e.g., eNB) typically have to check whether the carrier (i.e., the carrier frequency) is busy using some form of ‘listen before talk’ (LBT) mechanism, then an LTE device can begin transmissions only if the carrier is free. LBT typically comprises measuring the energy on the carrier (sometimes referred to as sensing) for a short duration (e.g., 9 us or 20 us) and determining whether the measured energy is less than a threshold (e.g., −82 dBm or −62 dBm). If the energy is less than the threshold, the carrier is determined to be free. Some examples of LBT include the CCA-ED (clear channel assessment-energy detect) and CCA-CS (clear channel assessment-carrier sense) mechanisms defined in IEEE 802.11 specifications, CCA mechanisms specified in ETSI EN 301 893 specification etc.
And, second, transmissions on the carrier typically also have to follow discontinuous transmission requirements (DCT requirements), i.e., the LTE device can continuously transmit for Xms (e.g., X=4 ms as per some regulations, up to 13 ms for some other regulations), after which it has to cease transmission for some duration (sometimes referred as idle period), perform LBT, and reinitiate transmission only if LBT is successful. The device may perform LBT towards the end of the idle period.
As such, what is needed is a modification to certain transmissions of LTE signals and on various channels that enable efficient wireless network operation in both the licensed and unlicensed carrier spectrums.